This invention relates to fluidized bed combustors and more particularly to improve the aerothermodynamic characteristics in such combustors for use in burning solid or highly viscous fuels.
Fluidized bed combustors are characterized by use of a generally static bed of granular inert material which is maintained in a fluidized state by passage of airflow therethrough. Such fluidized beds have been found highly useful in burning of solids or very heavy fuel oils. More particularly, the solid fuel in a granular state is introduced into the fluidized bed where combustion occurs. Typically the operating range of such a fluidized bed when used as a combustor is relatively wide with respect to the superficial velocity of airflow therethrough. More specifically, such a fluidized bed is theoretically operational between the minimum superficial velocity which establishes incipient fluidization of the bed, up to a maximum superficial velocity wherein elutriation of the bed particles themselves would occur.
Upon consideration of other aerothermodynamic characteristics of such a combustor, practical limitations required to establish efficient combustion as well as overall fluidized bed operation dramatically affect the operating range of the fluidized bed in terms of permitted superficial velocities. For example, the combustion efficiency when burning solid fuels is not only the function of the available oxygen, but also the temperature of the oxidizing zone and residence time in the oxidizing zone. Combustion efficiency is normally determined in terms of the total hydrocarbons converted to heat energy as a percentage of the total amount of convertible hydrocarbons provided by the fuel. Complete combustion or conversion of the hydrocarbons to heat energy can be achieved in a shorter time but at the expense of higher peak temperature within this zone. Alternately, lower peak temperatures may be utilized if the residence time in the combustion or conversion zone is increased.
Continuous operation of such a fluidized bed also requires proper and adequate removal of burned particles and flyash. Without such scavenging the bed itself tends to grow in size and become contaminated, reducing combustion efficiency. However, the superficial velocities required within and above the fluidized bed zone to establish good scavenging of the burned ash particles outwardly from the combustor are substantially different than those velocities required for ideal residence time within the combustor zone. Accordingly substantially narrow operational limits of the superficial velocities of the gas stream are imposed upon such a fluidized bed combustor. Such narrow operational limits, in addition to inhibiting design variations for minimizing peak temperatures, also tend to impose complicated and expensive controls for proper, continuous bed operation.
In addition to efficient combustion efficiency as well as proper overall operation of the entire combustor, the combustor must also be designed to maximize transfer of heat to a useable form. More particularly, many such fluidized bed combustors are used to heat a cycle fluid such as steam which is then transported to remote locations to perform useful work. The factors of efficient heat transfer are not necessarily compatible with those factors promoting efficient heat generation itself. For example, while the heat transfer coefficient for solid-to-solid heat transfer is generally improved with increased residence time of contact between the solids, heat transfer by convection is enhanced by increased gas velocity.